Multiphoton curing processes are described in U.S. Pat. No. 6,855,478. In these processes, a layer of material including a multiphoton curable photoreactive composition is applied on a substrate (e.g., a silicon wafer) and selectively cured using a focused source of radiant energy, such as an ultrafast laser beam. A multiphoton curing technique may be useful for fabricating two-dimensional (2D) and/or three-dimensional (3D) structures with micro- or nano-scale resolution.
In one fabrication technique, a voxel, or 3D volume element, is created when a pulsed laser beam of visible or near-infrared (NIR) radiation is focused into an engineered photopolymer resin. A non-linear interaction process within the resin initiates cure of the resin near a focus of the laser beam, where two photons of the NIR radiation are absorbed substantially simultaneously. The curing of the resin may be referred to as “photopolymerization,” and the process may be referred to as a “two-photon photopolymerization” process. Photopolymerization of the resin does not occur in regions of the resin exposed to portions of the NIR radiation having an insufficient intensity, that is, an intensity lower than a threshold dose for initiating photopolymerization.
A 3D structure may be constructed voxel-by-voxel with a multiphoton photopolymerization process by controlling a location of the focus of the laser beam in three dimensions (i.e., x-axis, y-axis, and z-axis directions) relative to the resin. In many cases, 3D structures are formed by curing approximately single voxel layers (e.g., in the x-y plane), followed by moving the focal point about one voxel length (e.g., in the z-axis), and curing a subsequent layer (e.g., in the x-y plane). This process may be repeated until the desired structure is at least partially cured.
Typically, the focal point of the laser beam is approximately spherical or ellipsoidal, with an intensity profile that is roughly Gaussian along any diameter. Accordingly, the voxels cured by exposure to the laser beam are roughly spherical, or may be similar to an elongated sphere, where the elongation is along one or more than one axis (e.g., x-axis, y-axis, or z-axis).